how to render in user perspective and not camera perspective
There is no real way to do that. (For example many settings like defocus depend on the camera object.) You simply will have to move the camera around.

Firstly: You can move the camera around like any object (grab with G, rotate with R and scale with S), but it is easier to go to the camera perspective (numpad 0) and enter fly mode (Shift+F). This allows you to position the camera in a more practical way. (See this in more detail here: https://youtu.be/xcZmyD9jkI4)

Secondly: If you want to save the camera's position for some reason, you can simply add a new one to the scene (Shift+A) and set it to be the active camera by pressing the Ctrl + 0 hotkey. Now the scene will be rendered from it and hitting 0 will make you look through it. You can tell which camera is active by looking at the camera object from another view: the small triangle above it is filled when it's active.

Thirdly: If your user perspective is "just the right way", you can align the camera to it (I can't remember the hotkey, but hitting space and looking for something along the lines of "Align camera to view" should get it).